Electrospray Ionization

A Brief History

The observation of liquid atomization and deformation in the presence of a strong electric field dates back several centuries, the underlying theory of which was first described by Lord Rayleigh as early as 1882 [14]. It would be more than 30 years before work was undertaken by John Zeleny to gain a better understanding of this phenomenon and take the first photographs of different “modes” of electrospray from small cylindrical emitters [15, 16]. Nearly 40 years after Zeleny’s observations, Sir. Geoffrey Ingram Taylor developed a theoretical model of the shape a deformed liquid meniscus assumes at the end of a capillary in the presence of an electric field, now known as a “Taylor cone” [17]. These studies laid the foundation for electrospray ionization (ESI), which would later prove to be an invaluable tool spanning many disciplines.

Around the same time Taylor was developing underlying theories of electrospray generation, Dole et al. [18, 19] showed that beams of fairly large (51-411 kDa) polystyrene ions could be generated by sampling an electrosprayed solution into vacuum. While not able to measure m/z of the ions directly, with a thorough understanding of ion-neutral interaction forces and by adjusting the potential of a charged repeller grid, they were able to verify the formation of a beam of “macroions” rather than low molecular-weight species [18], a fact confirmed by later experiments for the analysis of electrophoretic mobilities of the electrosprayed solutions [20].

Electrospray as a tool for mass spectral analysis was driven to the forefront of analytical science through seminal work published by Fenn et al. [21, 22] in laboratories at Yale University and later Virginia Commonwealth University. His, and the subsequent work of others demonstrated the ability of ESI to produce gas-phase ions with virtually no fragmentation, even for large biologically relevant species such as proteins and oligonucleotides [23], thus making them amenable to characterization by MS. From these footings, ESI coupled with MS (ESI-MS) became a standard tool for characterization of molecules and mixtures that required “gentle” ionization.

Initially ESI emitters used by Dole and Fenn were stainless steel hypodermic needles with either a tapered or beveled tip [18, 21]. Typically the diameter of these tips was 100-200 ^m with solution flow rates in the range of 1-15 ^L/min and electrospray potentials of up to 2-5 kV. Later, Mann et al. [24] would demonstrate the benefits of using an emitter formed by pulling a glass capillary to a tapered tip with a diameter of about 1 ^m. This design allowed for the use of lower volumes (several p.L), operation without a syringe pump, as well as no sheath or drying gas for coupling with MS. The move to smaller emitters also improved detection efficiency by a factor of 510, a property largely attributed to the low flow rate (< 50 nL/min) and initial droplet size of what would come to be known as nanoelectrospray ionization (nanoESI) [25].

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